1. Field of the Invention
The present invention relates generally to systems for transmitting and receiving digital signals, and in particular, to systems for broadcasting and receiving digital signals using hierarchical modulation techniques.
2. Description of the Related Art
Digital signal communication systems have been used in various fields, including digital TV signal transmission, either terrestrial or satellite.
As the various digital signal communication systems and services evolve, there is a burgeoning demand for increased data throughput and added services. However, it is more difficult to implement either improvement in old systems and new services when it is necessary to replace existing legacy hardware, such as transmitters and receivers. New systems and services are advantaged when they can utilize existing legacy hardware. In the realm of wireless communications, this principle is further highlighted by the limited availability of electromagnetic spectrum. Thus, it is not possible (or at least not practical) to merely transmit enhanced or additional data at a new frequency.
The conventional method of increasing spectral capacity is to move to a higher-order modulation, such as from quadrature phase shift keying (QPSK) to eight phase shift keying (8 PSK) or sixteen quadrature amplitude modulation (16 QAM). Unfortunately, QPSK receivers cannot demodulate conventional 8 PSK or 16 QAM signals. As a result, legacy customers with QPSK receivers must upgrade their receivers in order to continue to receive any signals transmitted with an 8 PSK or 16 QAM modulation.
Techniques have been identified for modifying the basic modulated QPSK signal to higher order modulation techniques (e.g. 8 PSK) to allow additional data to be transmitted and received by upgraded or second generation receivers. These techniques are also backwards-compatible. That is, they allow legacy receivers to receive and process the same basic QPSK signal essentially as if the additional data was not present. One such technique is hierarchical modulation. Hierarchical modulation is a technique where the standard 8 PSK constellation is modified to create a “non-uniform” 8 PSK constellation that transmits two signals (1) a QPSK signal that can be configured so as to be backwards-compatible with existing receivers, and (2) a generally more power efficient, non-backwards compatible signal. The backwards-compatible QPSK signal can be used to transmit high priority (HP) data, while the non-backwards-compatible signal can be used to transmit low priority (LP) data. While the HP signal is constrained to be the legacy signal, the LP signal has more freedom and can be encoded more efficiently using an advanced forward error correction (FEC) coding scheme such as a turbo code.
The application of conventional hierarchical demodulation techniques can result in excessive symbol errors in the LP data signal. Such errors can occur because of the excessive tracking errors in the timing/carrier recovery loop used in demodulating HP data signal, and in excessive symbol errors from the demodulated HP data signal.
What is needed is a system and method for receiving hierarchically modulated symbols, such as in non-uniform 8 PSK, that reduces LP data signal errors and provides for improved performance. The present invention satisfies that need.